fix(subagents): preserve restored hidden tabs safely

packages/protocol/src/client-capabilities.ts

@@ -14,6 +14,10 @@ export const CLIENT_CAPS = {
   // COMPAT(providerSubagents): added in v0.1.107. The daemon emits provider-owned
   // child descriptors and timelines only to clients that understand the new messages.
   providerSubagents: "provider_subagents",
+  // COMPAT(providerSubagentRetainTimeline): added in v0.1.108. Retained-remove
+  // updates are withheld from older clients because they interpret every remove
+  // as destructive and blank already-open provider child tabs.
+  providerSubagentRetainTimeline: "provider_subagent_retain_timeline",
   browserHost: "browser_host",
 } as const;
 

packages/protocol/src/messages.provider-subagents.test.ts

@@ -115,4 +115,40 @@ describe("provider subagent protocol", () => {
       }),
     ).toMatchObject({ payload: { kind: "remove", retainTimeline: true } });
   });
+
+  test("accepts a retained descriptor with provider timeline history", () => {
+    expect(
+      SessionOutboundMessageSchema.parse({
+        type: "agent.provider_subagents.timeline.get.response",
+        payload: {
+          requestId: "request-1",
+          parentAgentId: "parent-1",
+          subagentId: "child-1",
+          provider: "codex",
+          subagent: {
+            id: "child-1",
+            parentAgentId: "parent-1",
+            provider: "codex",
+            title: "Finished child",
+            description: null,
+            status: "completed",
+            createdAt: "2026-07-12T10:00:00.000Z",
+            updatedAt: "2026-07-12T10:01:00.000Z",
+            toolCallId: null,
+          },
+          hiddenFromTrack: true,
+          direction: "tail",
+          epoch: "epoch-1",
+          reset: false,
+          staleCursor: false,
+          gap: false,
+          window: { minSeq: 0, maxSeq: 0, nextSeq: 1 },
+          hasOlder: false,
+          hasNewer: false,
+          rows: [],
+          error: null,
+        },
+      }),
+    ).toMatchObject({ payload: { hiddenFromTrack: true, subagent: { id: "child-1" } } });
+  });
 });

packages/protocol/src/messages.ts

@@ -3093,6 +3093,8 @@ export const ProviderSubagentTimelineResponseMessageSchema = z.object({
     parentAgentId: z.string(),
     subagentId: z.string(),
     provider: AgentProviderSchema.nullable(),
+    subagent: ProviderSubagentDescriptorPayloadSchema.optional(),
+    hiddenFromTrack: z.boolean().optional(),
     direction: z.enum(["tail", "before", "after"]),
     epoch: z.string(),
     reset: z.boolean(),
@@ -4877,6 +4879,7 @@ export const WSHelloMessageSchema = z.object({
       [CLIENT_CAPS.customModeIcons]: z.boolean().optional(),
       [CLIENT_CAPS.terminalReflowableSnapshot]: z.boolean().optional(),
       [CLIENT_CAPS.providerSubagents]: z.boolean().optional(),
+      [CLIENT_CAPS.providerSubagentRetainTimeline]: z.boolean().optional(),
       [CLIENT_CAPS.browserHost]: BrowserAutomationHostCapabilitySchema.optional(),
     })
     .passthrough()